In a typical lost foam casting process, a foam mold pattern is placed within a mold flask, wherein the mold pattern includes a foam riser that extends from the pattern towards the top of the flask. Sand from a hopper located above the flask is poured into the flask about the pattern. As the sand fills the flask, the sand becomes compacted about the pattern, forming a mold cavity. After the flask has been adequately filled with sand, which preferably corresponds to a level equal to the top of the riser, molten metal is poured through the riser into the mold cavity, which vaporizes the foam riser and pattern. Thus, the molten metal replaces the foam pattern. The metal is cooled until the casting is solidified, at which time the casting and sand are removed from the flask.
A problem that has been encountered in the industry is that if sand is not uniformly distributed about the foam pattern during the sand filling process, the pattern is subjected to uneven weight distribution from the sand which can damage or distort the pattern, resulting in an inferior or unusable casting. Various improvements to address this problem have been implemented with varying degrees of success. One such improvement is the use of sand distribution plates to promote uniform distribution of sand in the flask, wherein a first distribution plate ("fixed plate") having a plurality of apertures therethrough is affixed to the bottom of the hopper and a second distribution plate ("slide plate") having a plurality of apertures therethrough, corresponding to the apertures through the first plate, is slidably mounted subjacent the first plate such that the second plate is movable between a closed position, wherein the apertures in the first and second plates do not overlap, and an open position, wherein the apertures in the first and second plates overlap such that multiple streams of sand "rain" into the flask. The apertures through the plates can be fully overlapped for maximum sand flow, or they can be partially overlapped to infinitely variable degrees for reduced sand flow. This is an important feature because when sand is initially poured into the flask, the flow rate should be reduced so that the sand (1) will not damage the pattern and (2) will have adequate time to fill any cavities in the pattern. As the sand level rises above the top of the pattern, the apertures through the distribution plates are preferably fully overlapped for maximum sand flow to quickly fill the remainder of the flask, thereby maximizing production efficiency.
While use of the sand distribution plates, collectively known in the art as a "rain gate" or "sand gate", has substantially improved uniform sand distribution in the flask, other problems have arisen. First, when the apertures in the distribution plates are partially overlapped, the sand flow therethrough is (1) skewed such that the sand tends to accumulate towards one side of the flask rather than uniformly therein and (2) offset from the center of the apertures through the fixed plate, thereby shifting the predetermined streams of sand such that the sand tends to accumulate towards one side of the flask rather than uniformly therein. The non-uniform sand accumulation results in uneven weight distribution about the pattern which can damage or distort the pattern, resulting in an inferior or unusable casting. Second, sand can accumulate between the plates and between the slide plate and the supports therefor. If this occurs, as the sand gate is opened and closed, abrasion from the sand will slowly erode the plate surfaces until gaps form between the plates, resulting in (1) greater sand accumulation between the plates and between the slide plate and the supports therefor, thereby increasing the erosion, (2) increased skewed and offset sand flow during the reduced sand flow portion of the filling cycle, and (3) greater sand flow than predicted at any particular overlapped position during the reduced sand flow portion of the filling cycle.
Accordingly, what is needed is an improved sand gate design to promote uniform sand distribution in a mold flask which overcomes the problems in the prior art.